1. Field
Technology for interconnecting integrated circuitry, packages and printed circuit boards.
2. Background
In metallurgy, “wetting” refers to the ability of a liquid to maintain contact with a solid surface. Wetting is an important factor in bonding two materials. When bonding a circuit package with a carrier (e.g., a printed circuit board), solder paste may be used to temporarily hold the solder balls (or solder bumps) on the circuit package to the board. A reflow process can be performed to melt the solder balls (or solder bumps), which form solder joints after the molten solder is cooled. The solder joints provide an electrical and mechanical connection between the package and the board.
During the reflow process, circuit packages often experience various degrees of warpage as the temperature changes. The changing degrees of warpage can be referred to as “dynamic warpage.” Dynamic warpage is often the cause of non-wet joints, which means the solder balls fail to properly adhere to the surface of the board. For example, during the surface mount (SMT) reflow, dynamic warpage usually results in insufficient time for the flux to clean off the oxide and residue layer on the solder balls to form proper solder joints. As such, the formation of non-wet solder joints may occur.
Further, dynamic warpage can result in an inadequate flux and solder ball contact. The inadequate flux and solder ball contact can cause non-uniform collapse of the solder balls during the reflow. Thus, solder balls that are not in contact with the board during a period of reflow may have an increased oxide surface layer. When the warpage changes to force these solder balls back in contact with the board, adjacent solder balls may be squeezed out and form a bridge, which is referred to as a solder bump bridge.
On a surface having poor wetting characteristics (e.g., a heavily oxidized solder surface), molten solder may spread during reflow and two adjacent solder bumps may melt into each other to form a solder bump bridge. Solder bump bridging may be the cause of a short circuit.
FIG. 1 illustrates an example of a surface mount (SMT) reflow process 180, in which a circuit package 100 is to be electrically and mechanically bonded to a print circuit board 120 through an array of solder bumps 130. During the reflow, mismatch in thermal expansions causes circuit package 100 to warp. For example, circuit package 100 may initially bulge at the center and then bend into a saddle shape as the heat increases. Solder bumps 130 are oxidized during the reflow, which act as a barrier that prevents proper wetting. As a result, the formation of a non-wet joint 150 may occur. Further, a solder bump bridge 160 may be formed as a result of the dynamic warpage. The non-wet joint and solder bump bridging can result in failures of the circuit package or the board.
Conventionally, the problem of non-wet joint and solder bump bridging is solved by modifying the reflow process. For example, the volume of solder paste can be increased, or the heat profile of the reflow can be modified. However, the conventional solutions have had minimal success, with an increased risk for other issues, such as solder bridging.